Potentially misleading figures about barristers' earnings were released by the Government to justify its cuts to legal aid, the statistics watchdog has warned.

In a critical letter to the Ministry of Justice, the head of the UK Statistics Authority (UKSA), Sir Andrew Dilnot, said the department had effectively neglected lower estimates of barristers' average earnings in favour of higher calculations of £84,000 a year.

Figures published by the MoJ in January represented the mean fee income from public funds for full-time barristers, where any barrister with a fee income of less than £10,000 is excluded, the UKSA said. Calculations also included VAT, which barristers must pay to the taxman , and expenses such as travel costs.

Explanatory notes "critical to understanding of the statistics" made it clear the figures did not represent the earnings of individuals; however, the watchdog found they are not "clearly referenced" in the release.

Shadow justice secretary Sadiq Khan said: "This embarrassing slap-down shows that, when ministers attempt to use figures to support their flimsy arguments and policies, they can't be trusted.

"If the statistics which the Tory-led Government is using to justify massive upheaval to legal aid are dodgy, then serious questions are raised. Is it any wonder that those who work in the justice system believe that you can't trust anything the Ministry of Justice says?"

Ministers released the figures a few days before barristers and solicitors staged their first walkout in protest at cuts to fees.

Justice Secretary Chris Grayling is pressing ahead with reforms in a bid to slash £220 million from the legal aid budget by 2018/19.

Ministry of Justice officials have previously argued hat the final reforms reflect many of the changes barristers and solicitors have asked for and also introduce measures to support lawyers through the period of transition.

In his letter addressed to courts and Legal Aid Minister Shailesh Vara, Sir Andrew said: "The statistical report focuses on the 'fee income' of barristers but does not clearly define this term.

"The commentary talks in places about the 'earnings' of barristers.

"This is potentially misleading as the 'Important notes to the data' make it clear that the statistics do not represent the earnings of individuals.

"Information about items included or excluded from the estimates of fee income - for example, VAT and disbursements - is also presented in the notes but the report makes no attempt to quantify the impact of the treatment of these items.

"The notes are critical to understanding of the statistics but they are not clearly referenced alongside the data table.

"The report also presents mean and median measures, but does not advise users about the appropriate use of these different measures."

Hundreds of lawyers marched on Westminster earlier this month in protest at the cuts while thousands of barristers staged a second nationwide walkout.

Nicola Hill, president of the London Criminal Courts Solicitors Association, which organised the march and walkout last week, said: " This comes as no surprise to those fighting to save criminal legal aid.

"The Ministry Of Justice repeatedly states that the cost of legal aid is far higher than the statistics show.

"Another worrying aspect of this casual approach to facts and figures can be seen in government claims about the amount of savings that need to be made in the criminal justice system.

"The MoJ has consistently ignored year on year savings for the last ten years as a result of falling crime. We would like to engage with a Lord Chancellor who deals in fact not fiction."

An MoJ spokeswoman said: " We stand by the figures we used.

"Whether the mean or median figure gives the best representation of typical fee income is open to statistical interpretation.

"We also published the full range of fee incomes received by barristers in 2012/13, including both the mean and median fee income figures and underlying source data, to be as transparent as possible.

"This is all publicly available on the Government's website. We agree that this fee income is not the same as individual earnings, and this was stated in the statistical report we published on January 2."

Nigel Lithman, chair of the Criminal Bar Association, which also organised the march and walkout last week, said: "The Criminal Bar Association has been battling with the Ministry of Justice over its statistics for more than a year.

"We claim that the need to cut our fees is not made out and the figures issued are misleading.

"We also claim that the figures of the average earnings of member of the Criminal Bar have been grossly exaggerated in a manner to make the public hostile towards what they are encouraged to regard as an overpaid profession.

"Prior to a day of national protest by the Bar, the MoJ chose to issue the fanciful figure that the average earnings were £84,000 per annum.

"For some reason they ignored the lowest earners, included VAT and excluded expenses.

"They chose to slew the figures. The true figures were the Criminal Bar earning no more on average than £34,000 before tax and many on far less.

"Our cries went unheard, until now.

"But it seems the concerns are taken up by the UK Statistical Authority who begin to take the MoJ to task for potentially misleading us.

"This is an outrage and frankly the public and the profession have a right to expect more from a Ministry of Justice. This is Britain."